Autoimmune thyroid diseases and nonorgan‑specific autoimmunity.

Autoimmune thyroid diseases (ATD), as the most common organ‑specific autoimmune disorder, is frequently accompanied by other organ- and nonorgan‑specific autoimmune diseases. Although the exact pathogenic mechanism of the coexistence of autoimmune disorders has not been clearly defined, genetic and environmental factors, immune defects, and hormonal changes, may play a key role in polyautoimmunity. The role of human leukocyte antigen (HLA) haplotypes, HLA-B8 and -DR3, in the overlapping of autoimmune disorders was well supported by higher frequency of these haplotypes in primary Sjögren's syndrome (PSS) and ATD. In addition, polymorphisms of the cytotoxic T lymphocytic antigen 4 gene have been reported to be associated with many autoimmune disorders especially those coexisting with ATD. Definite noncasual association of ATD has been clearly documented in patients with PSS, rheumatoid arthritis, and systemic lupus erythematosus. Possible association with ATD is also considered in systemic sclerosis and dermatomyositis. Many authors documented a significantly higher prevalence of antinuclear antibodies (ANAs) in ATD patients in comparison with controls; however, the clinical significance of ANAs in this group is still unknown. The presence of other non‑organ‑specific antibodies has not been convincingly demonstrated. On the other hand, the prevalence of antithyroid antibodies as well as ATD is higher in patients with systemic connective tissue disease compared with the general population. Basedon these data, there is no evidence for the utility of ANA testing in patients with ATD, but because of the high prevalence of ATD and antithyroid autoantibodies, it is clinically important to screen patients with autoimmune rheumatic disorders for the presence of thyroid autoimmunity.